A potential induced degradation (PID) effect refers to a phenomenon that an output performance of a photovoltaic module is degraded when an output terminal of the photovoltaic module (i.e., a string input of a photovoltaic inverter system) suffers a high negative bias voltage relative to a metal frame of the photovoltaic module. The PID effect is characterized in reducing of an open-circuit voltage, a short-circuit current and a fill factor of the module. In recent years, with fast development of a photovoltaic grid-connection power generation system, a string input voltage configuration of the photovoltaic inverter system is higher and higher, and a system of 1500V is spread and applied gradually. Since the metal frame of the photovoltaic module is generally grounded, under a high string input voltage, a high negative bias voltage is formed between a battery panel and the grounded metal frame in a photovoltaic module close to a negative electrode terminal of the string, such that charges transfer occurs, and a surface of the module is polarized. The photovoltaic module closer to the negative electrode terminal of the string has more obvious polarization (as shown in FIG. 1). Under the negative bias voltage for a long period, particularly in a high temperature and high humidity condition, a serious PID effect will occur in the photovoltaic module, resulting in reducing of a power generation capacity of the system year by year.
The current photovoltaic module manufacturer prevents the PID effect of the module mainly by using packing materials with high performances, which has a high cost and is not adaptive to a built photovoltaic power station. For a built photovoltaic system, the PID effect is prevented normally by a solution of grounding of a negative electrode of the photovoltaic string or a solution of raising a potential of a virtual neutral point at an alternating current (AC) power grid side. In the solution of grounding of the negative electrode of the photovoltaic string, the negative electrode of the photovoltaic string is directly grounded, such that potentials to ground of series-connected battery panels in the string are always positive bias voltages, thereby preventing PID effects. In the solution of raising a potential of the virtual neutral point at the AC power grid side, a potential to ground of the virtual neutral point at the AC side is raised by an external direct current source, such that a potential to ground of a neutral point of a direct current input bus is raised, thereby indirectly raising a potential to ground of the negative electrode of the input module above a zero potential. However, the solution is only adaptive to a case that the inverter is grid-connected.
The PID effect reduces a power generation capacity of the photovoltaic battery panel, and a cost of the conventional PID effect preventing solution is high. The above two solutions for the PID effect can only prevent the PID effect to a certain degree and cannot repair the photovoltaic module in which the PID effect occurs.